lipid
Lipids are fatty acid esters, a class of water-insoluble organic molecules, that are used as basic building blocks of biological membranes, as well as for energy storage (e.g., triglycerides). Lipids consist of a polar or hydrophilic (attracted to water) head and one to three nonpolar or hydrophobic (repelled by water) tails. The hydrophobic tail consists of one or two (in triglycerides, three) fatty acids. These are unbranched chains of carbon atoms (with the correct number of H atoms), which are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.